1. Field of the Invention
The present invention falls in the area of heat treatment and thermomechanical treatment of certain products, in particular deep-drawn sheet metal parts.
2. Description of the Related Art
Spatially shaped parts from metal sheet are typically produced by deep drawing. This procedure is characterized in that a metal sheet with a suitable easy-to-deform microstructure is held by a blank holder and shaped and drawn over the edge of a female drawing die by a male punch. The shape of the product is governed by the shape of the tools. The cold forming process causes the material to work harden. The higher the yield strength of the feedstock, the greater is the springback effect. Springback causes problems with production precision and repeatability. This is why a new hot drawing process was recently developed, which includes hot drawing and press hardening. This process involves processing of metal sheet where the blank is heated to the austenite region, drawn in austenitic condition and, thanks to rapid heat transfer to the female drawing die, cools down between the tools in such a way that hardening microstructure, most often martensite, is obtained in the formed part. This leads to smaller dimensional variation resulting from springback. The typical material used in this application is 22MnB4, the strength of which is about 1,500 MPa after quenching.
It is in particular the safety components in automotive industry which require the highest possible value of the product of strength and elongation in order for the components to be able to absorb as large as possible amount of the impact energy by deforming at high flow stress and without premature instability and fracture failure. These requirements are met in part through suitable engineering design of the components and by the use of metal sheets of dissimilar thickness. Feedstock of this type can be made by welding together sheets of dissimilar thickness, the so-called tailored blanks, or by rolling sheets to obtain variable thickness along the length of the strip. Such sheets are used as blanks for both cold and hot deep drawing. In order to improve the state of the art while keeping or even reducing the resulting weight, the properties of materials need to be improved in a comprehensive manner: particularly enhancing the strength and seeking methods for improving elongation.